We actually do have some sort of an explanation. *sort of
There’s not a lot of soulcasters.
Out of the Elsecallers it’s just Jasnah, and while I’m pretty sure we only saw less than 20 Lightweavers. Based on Shallans comments on how she was glad the Lightweavers stayed small, I’m 90% sure that the Unseen Court consists of all the Lightweavers at the time.
And said soulcasters were almost always busy doing something else. The strategic value of the maps, and their illusions does mean you can’t have them always soulcasting. There are a lot of other ways to use a Lightweaver.
Now, if they did do a rotation of always having a radiant or two soulcasting, I don’t think they could get infinite food. Soulcasting takes effort from the radiant, and they need to take breaks. Also, the bigger something is, the harder to soulcast. Basically, even if they had all the light weavers and soulcasters on it, I don’t think they’d be able to keep up with the food demand of Urithiru.
And finally. Y’all. They had access to Towerlight for less than 10 days. And they had a war to fight. I highly doubt infinite food glitches were on their mind.